Isn't the url always added automatically to the forum post? It is for my site
On the rest you can do this already, though the excerpt does need to be added manually when the article is posted (a simple copy and paste?)
Use a new field called say 'teaser' or something and make sure that you fill in the 'Topic Format'
You can also add a character restriction to this field if needed
Then remove the 'Topic Format' from your original content field.
Then when posting the article fill in the 'excerpt' manually (copy and paste)
The above should then, when publishing an article, post as well a forum post containing an 'excerpt' (the teaser field) and a link to the article